1. Field of the Invention
The present invention relates to a fixing member, a method for producing a fixing member, and a fixing device using the fixing member.
2. Description of the Related Art
Conventionally, electrophotographic image forming apparatuses, e.g. copiers, printers and facsimiles, generally include photoconductor drums which rotate, and a photosensitive layer of each photoconductor drum is uniformly charged and then exposed to a laser beam coming from a laser scanning unit so as to form a latent electrostatic image thereon. The latent electrostatic image is developed with toner and then transferred onto transfer paper that is a recording medium. Subsequently, the transfer paper is passed through a thermal fixation device where the developed image is thermally fixed on the transfer paper (thermal fixing method).
In full-color copiers and laser printers, toners of four colors, i.e. magenta (M), cyan (C), yellow (Y) and black (K), are used. When a color image is thermally fixed, it is necessary to mix these color toners in a melted state, so that there is a need to make the toners lower in melting point such that the toners can easily melt and to uniformly mix the several types of color toners in a combined manner and in a melted state on the surface of a heating and fixing roller.
In the thermal fixing method, since a toner image fused with a recording medium such as paper comes into contact with a fixing member, the outermost layer of the fixing member is formed of a material (e.g. fluorine resin) which is superior in separability. However, even when such a fixing member is used, the melting toner is liable to adhere to the surface of the fixing member owing to its softness and high viscosity, so that the winding of the recording medium such as paper may arise.
In recent years, demands for resource saving and energy saving have been heightening to protect the earth's environment, and there has been a tendency to reduce the melting point of toner in electrophotographic image forming apparatuses to save power consumption in accordance with the energy saving. Attempts are being made to use resins having relatively low molecular weights in order to reduce the melting point of toner; however, the toner becomes stickier when it melts, and thus a recording medium is more likely to be wound around the fixing member.
Accordingly, dispersion of a larger amount of wax in the toner, application of a larger amount of oil to the fixing member, and so forth are being considered to prevent the winding.
However, problems of serious side effects such as the following have been pointed out: when a large amount of wax is contained in the toner, the wax is highly likely to be exposed at the surface of the toner, which causes roller filming and/or a carrier-spent phenomenon; moreover, since a large amount of low-viscosity wax is mixed in the toner, offset arises owing to a reduction in the cohesive force of the overall toner. Although the problems such as roller filming and a carrier-spent phenomenon can be lessened by making a polymerized toner, etc. have a multilayer structure, the problem of the reduction in cohesive force cannot be prevented.
Further, there is a problem of smearing; for example, application of a large amount of oil can make paper sticky. This problem can be solved by reducing the amount of the oil applied; however, there is such a problem that when the amount is reduced in this manner, the oil is repelled by a release surface subjected to image fixation, the repelled oil is formed into the shape of dots and so cannot uniformly cover a fixation surface, and thus releasing effects are nullified.
Other known measures to prevent the winding include (1) a technique of feeding paper in the opposite direction to the fixation surface at the time of separation; and (2) a mechanical separation method such as addition of a member which induces forceful separation.
In using the member which induces forceful separation, there is generally a method employed in which a member called “separation claw” is brought into contact with a fixing member so as to prevent paper from being wound around the fixing member.
However, if the separation claw is brought into contact with the fixing member in this manner, the contact portion of the separation claw may scratch the fixing member, so that scratches may be transferred in a fixing step and an abnormal image may be thereby formed. Accordingly, ways of reducing the damaging behavior of the separation claw to the fixing member, for example by providing the separation claw with a fluorine-based resin surface layer superior in slidability or rounding angles in the vicinity of the contact portion of the separation claw, have been devised; however, when paper powder produced during printing is sandwiched between the separation claw and the fixing member, the paper powder is pressed against the fixing member by the separation claw, which too causes scratches, and thus an abnormal image may arise.
In recent years, along with the colorization, there has been a method of fixing toners of several colors, laid on top of one another over a recording medium, to the recording medium, in which a fixing member is used that includes a belt made of a polyimide or metal, an elastic layer made of silicone rubber or the like formed on the belt, and an adhesion preventing layer made of fluorine resin or the like to prevent adhesion of the toners. However, this fixing member is problematic in that a separation claw presses into the elastic layer further, thereby easily damaging the fixing member.
To solve the above-mentioned problems, there has, for example, been proposed a method (separation plate method) (refer to Japanese Patent Application Laid-Open (JP-A) No. 2001-83832), which is a combination of a technique of feeding paper in the opposite direction to the fixation surface at the time of separation, and a member (separation plate) that promotes separation of a recording medium such as paper from the fixing member while not in contact with the fixing member. Use of such a separation plate makes it possible to promote separation between the recording medium and the fixing member without scratching the fixing member.
In the separation plate method, however, an initial separating function is dependent upon the elasticity of paper, so that when the linear velocity is high, initial separation often does not take place in time. In other words, when the recording medium such as paper is electrostatically or chemically attached to the fixing member, there is a great reduction in separating function, and the separating performance tends to be insufficient especially at high linear velocity. In the case where a copy pattern with a large amount of adhesive toner is used, initial separation does not take place in time, which causes an image portion to touch the separation plate, and thus an image defect such as formation of streaks arises. Consequently, as the linear velocity increases, there is a paper jam caused on the separation plate. For that reason, the separation plate method is selectively used for copiers of low linear velocity and copiers of intermediate linear velocity.
Meanwhile, Japanese Patent (JP-B) No. 4015785 describes improvement in surface adhesion by forming fluorine resin protrusions on a fluorine rubber surface layer; however, serious temporal deformation arises owing to abrasions to the protrusions caused by the contact with recording media, dust, etc., and thus long-term stable effects cannot be obtained.
JP-A No. 2007-316529 describes provision of depressions capable of holding wax components as a release agent of toner; however, since the shape is obtained merely by processing the surface of a single member (having an equal hardness), a change cannot be produced in wettability, and thus the wax holding force is weak. Also, in some cases, the fixed surface shape is transferred to an output image, thereby reducing the glossiness of the image and lowering the image quality.
Therefore, in reality, the following have not yet been provided: a fixing member which surely promotes separation of a recording medium from the fixing member even at high speed, which can reduce the incidence of image defects and paper jams and form a high-quality image, and which undergoes less temporal deformation; a method for producing a fixing member; and a fixing device using the fixing member, which is capable of realizing stable image fixation for a long period of time.